_common.styl: unify #donate a font-family (#568)

* _common.styl: unify #donate a font-family

* Improve style of donate button for $body-font

Author: JLHwung

themes/vue/source/css/_common.styl

@@ -194,11 +194,12 @@ a.button
     border: 1px solid #d4d4d4
     img
         display: inline-block
-        vertical-align: middle
+        vertical-align: bottom
         width 14px
         height 14px
     a
-        font 700 11px/14px 'Helvetica Neue', Helvetica, Arial, sans-serif
+        font 700 14px/14px $body-font
+        margin-left: 2px
         color #333333
         text-shadow 0 1px 0 #fff
     &:hover